Total CO₂ Measurements

Section P16S_2005

The TCO₂ analytical equipment was set up in a seagoing container modified for use as a shipboard laboratory. The analysis was done by coulometry with two analytical systems (PMEL-1 and PMEL-2) operated simultaneously on the P16S_2005 cruise by Dr. Christopher Sabine (PMEL) and Ms. Justine Afghan (SIO). Each system consisted of a coulometer (UIC, Inc.) coupled with a Single Operator Multiparameter Metabolic Analyzer (SOMMA) inlet system developed by Ken Johnson (Johnson et al. 1985, 1987, 1993; Johnson 1992) of Brookhaven National Laboratory (BNL). In the coulometric analysis of TCO₂, all carbonate species are converted to CO₂ (gas) by addition of excess hydrogen to the seawater sample, and the evolved CO₂ gas is carried into the titration cell of the coulometer, where it reacts quantitatively with a proprietary reagent based on ethanolamine to generate hydrogen ions. These are subsequently titrated with coulometrically generated OH^-. CO₂ is thus measured by integrating the total charge required to achieve this.

The coulometers were each calibrated by injecting aliquots of pure CO₂ (99.995% purity) by means of an 8-port valve outfitted with two sample loops (Wilke et al. 1993). The instruments were calibrated at the beginning of each station with a set of the gas loop injections. Subsequent calibrations were run either in the middle or end of the cast if replicate samples collected from the same Niskin, which were analyzed at different stages of analysis, differed by more than 2 µmol/kg.

Secondary standards were run throughout the cruise on each analytical system; these standards are Certified Reference Materials (CRMs) consisting of poisoned, filtered, and UV-irradiated seawater supplied by Dr. A. Dickson, SIO, and their accuracy is determined onshore manometrically. On this cruise, the overall accuracy and precision for the CRMs on both instruments was -1.7±0.8 µmol/kg (n=63) and -2.4±0.7 µmol/kg (n=64) for PMEL-1 and PMEL-2 respectively. The final TCO₂ data reported to the database have been corrected to the Batch 67 CRM value.

Samples were drawn from the Niskin-type bottles into cleaned, precombusted 300-mL Pyrex bottles using silicone tubing. Bottles were rinsed three times and filled from the bottom, overflowing half a volume, and care was taken not to entrain any bubbles. The tube was pinched off and withdrawn, creating a 3-mL headspace, and 0.2 mL of 50% saturated HgCl₂ solution was added as a preservative. The sample bottles were sealed with glass stoppers lightly covered with Apiezon-L grease and were stored at room temperature for a maximum of 24 h prior to analysis.

TCO₂ values were reported for 2,882 samples or approximately 75% of the tripped bottles on this cruise. Full profiles were completed at odd-numbered stations on whole degrees, with replicate samples taken from the surface, oxygen minimum, and bottom depths. On the even-numbered (half degree) stations, as many samples as possible were drawn based on the current sample throughput; replicates were collected from the surface and bottom bottles. Typical even-numbered stations had between 8 and 20 bottles sampled.

Duplicate samples were drawn from 256 bottles and interspersed throughout the station analysis for quality assurance of the coulometer cell solution integrity. The average of the absolute value of the difference between duplicates was 1 µmol/kg for both systems. No systematic differences between the replicates were observed.

Section P16N_2006

The TCO₂ measurements on Section P16N_2006 were done by the coulometry with the same two analytical systems (PMEL-1 and PMEL-2) as on Section P16S_2005, operated simultaneously on the cruise by Bob Castle (NOAA/AOML) and Alex Kozyr (ORNL/CDIAC) on Leg 1 and Dana Greeley and David Wisegarver (both of NOAA/PMEL) on Leg 2 (see system and method descrition in Section P16S_2005 above). On this cruise, the overall accuracy for the CRMs on both instruments combined was 0.8 µmol/kg (n=66) for each leg. The final TCO₂ data reported to the database have been corrected to the Batch 73 CRM value.

The TCO₂ values were reported for 2,648 samples or approximately 80% of the tripped bottles on this cruise. Full profiles were completed at stations on whole degrees, with replicate samples taken from the surface, oxygen minimum, and bottom depths. Duplicate samples were drawn from 121 bottles on Leg 1 and 72 bottles on Leg 2 and interspersed throughout the station analysis for quality assurance of the coulometer cell solution integrity. The average of the absolute value of the difference between duplicates was 1 µmol/kg for both systems. No systematic differences between the replicates were observed.